Due to the increasing need for rapid matching capabilities, in networking hardware equipment for example, content addressable memories (CAMs) continue to proliferate. A CAM may perform matching functions by applying a search key or “comparand” to a table of stored data values. A CAM may then determine if any of the data values match a given search key.
CAM devices may take a variety of forms. As but a few of the possible examples, some CAM devices are based on particular types of CAM cells. Such cells may include storage circuits integrated with compare circuits. Examples of storage circuits may be static random access memory (SRAM) type cells or dynamic RAM (DRAM) type cells. Alternate approaches may include RAM arrays, or the like, with separate matching circuits and/or matching processes executed by a processor, or the like.
Conventional CAM devices may include both binary and ternary CAM devices. Binary CAM devices can provide a bit-by-bit comparison between a stored data value and a search key. Ternary CAM devices can provide maskable compare operations that can selectively exclude predetermined bits of a data value from a compare operation.
Typically, a conventional CAM device can generate match indications for each entry. That is, each entry can be compared with an applied search key value. If a search key value matches a stored data value, a match (or “hit”) indication may be generated for the entry. Conversely, if a search key value does not match a stored data value, a mismatch (or “miss”) indication may be generated for the entry.
Match results in a CAM device may include single match results, that can be generated when a single entry matches an applied key value, as well as multiple match results, that may be generated when more than one entry matches an applied key value.
While CAM entries can provide the above described match or search function, CAM entries may also include conventional data access functions, such as read or write operations, to read or write the data values that are compared to a search key.
Decoder circuits within CAM devices can conventionally select a CAM entry for a particular operation, such as a read or write. More specifically, conventional circuits may enable a particular CAM entry by coupling such a CAM entry to bit lines, or the like, so that data may be read from the entry or written into the entry. Mask data or similar data may be written for a given CAM entry in a similar fashion.
Conventional CAM decoder circuits can be conceptualized as being “one-hot” type decoders. That is, a typical conventional CAM decoder can receive a binary input value of N bits and output 2N pre-decode output values. In response to each particular input value, one of the pre-decoder output values can be activated.
While one-hot type decoders can be suitable for read and write operations in a CAM device, it may be desirable to employ decoders that can provide more advanced functions in order to provide additional features in a CAM device.